The present invention relates to a method for the deposition of a coating layer on an optical fiber while it is being drawn. This coating layer is designed to improve the imperviousness and lifetime of the optical fiber.
At present, optical fibers are widely used in long-distance transmission networks. They are also designed to convey information up to the home of a residential user. These fibers, made of silica, are nevertheless sensitive to ambient humidity which greatly weakens their fatigue strength and therefore reduces their lifetime.
One problem therefore lies in finding a solution for the low-cost protection of optical fibers against ambient humidity without causing deterioration in their mechanical strength. For this purpose, several types of so-called hermetically sealing coatings, with very low permeability to water vapor, have been designed and developed.
These hermetically sealing coatings make it possible to increase the fatigue strength of the fibers and therefore increase their lifetime.
The following relationship expresses the lifetime of a fiber as a function of the critical stress sc of the stress applied sa, and of a factor n expressing the permeability of the fiber:
tf=B* scnxe2x88x922/san; and B is a constant.
After measuring the lifetime tf, namely the rupture time as a function of the different stresses applied, the value n can be deduced by plotting the curve In tf=f(In sa).
It can be seen that the lower the permeability of the fiber to water vapor, the higher is the factor n and therefore the greater is the lifetime tf.
The various hermetically sealing coatings that have been studied up to now are made of ceramic, metal, carbon and hydrophobic polymer type materials.
The ceramics are for example silicon carbide, titanium carbide or silicon nitride. Their deposition on optical fibers is done according to the well-known technique of chemical vapor phase deposition (CVD). In this case, the speed of deposition associated with this process is in the range of 60 nm/min, which is far too small to be compatible with a dynamic fiber-drawing method in which the running speed of an optical fiber is generally greater than 20 m/min. Furthermore, the deposition of this type of coating is necessarily done on substrates heated to a temperature greater than or equal to 900xc2x0 C.
Metal coatings such as aluminum, zinc or tinplate for example are obtained by a method known as the xe2x80x9cfreezingxe2x80x9d method in which a molten metal fiber is impregnated. In no circumstance can this type of method be used during a fiber-drawing operation. For, the deposition speed is greatly limited by the cooling of the molten metal and is therefore not compatible with the fiber-drawing speed. The fibers coated with such coatings are used only for specific applications such as the manufacture of sensors or fibers for components for example, or they are used for their resistance to high temperatures in the case of metals such as aluminum, copper or gold for example. Furthermore, these metal coatings are necessarily deposited on substrates heated to temperatures greater than or equal to 900xc2x0 C.
The deposition of carbon has also been considered. This deposition uses xe2x80x9cdiamondxe2x80x9d type carbon and xe2x80x9cgraphitexe2x80x9d type carbon. This deposition is done according to the standard technique of chemical vapor deposition (CVD), under atmospheric pressure, on fibers placed in a reactor. The fibers obtained generally have high fatigue strength with a factor n of about 100, but their mechanical strength thereby deteriorates. Furthermore, the reagents used, which are of the hydrocarbon type, are present in large quantities and clog the walls of the reactor. Owing to this pollution, high quality deposits can be made only on very small fiber lengths and recycling procedures have to be frequently performed. Consequently, the cost of the fibers is considerably increased.
xe2x80x98Diamondxe2x80x99 type carbon is deposited on a fiber heated to 200xc2x0 C. or more. However, at this temperature, it is deposited simultaneously on the walls of the reactor. To prevent clogging of this kind, it is necessary to cool the walls of the reactor significantly with respect to the fiber. Now a cooling of this kind is very difficult to achieve at temperatures below 200xc2x0 C.
Finally, coatings of hydrophobic polymers have been designed. The deposition is performed identically to a standard deposition. However, the imperviousness of this type of coating diminishes constantly after exposure to humidity and it loses its hydrophobic properties after a few years.
The present invention makes it possible to mitigate the above-mentioned drawbacks since it proposes a low cost method that can be used to deposit a hermetically sealing coating on an optical fiber at very high speed, compatible with the fiber-drawing method, without causing deterioration in the mechanical properties of the fiber.
It pertains more particularly to a method for the deposition of a coating layer on an optical fiber while it is being drawn, this coating layer being designed to improve the imperviousness and lifetime of the optical fiber, characterized in that the method consists in carrying out a decomposition of a gas mixture of boron halogenide and hydrogen and/or boron halogenide and ammonia gas by means of a microwave plasma-assisted addition of energy, and in that the operation is conducted in the presence of a carrier gas in order firstly to carry the gas mixture towards a reaction medium and secondly to activate the plasma.
Boron is next to carbon in the periodic table of elements and therefore has properties similar to those of carbon. Thus, it has a highly covalent type of chemistry. Furthermore, its electron structure explains its small atomic radius, its high density and its very great hardness. It also has a low expansion coefficient and a high softening point. Boron nitride for its part is also very hard and is generally used to improve the abrasion strength and resistance to wear and tear of certain objects.
The obtaining of boron by the reduction of boron halogenides such as chlorides and bromides under hydrogen is well known as the Van Arkel reaction. Similarly, boron nitride is obtained by the reduction of boron halogenide in the presence of ammonia gas. In general, the energy needed for the chemical reaction is provided either by heating during an operation of chemical vapor deposition (CVD) or by plasma-assisted heating during an operation of plasma-assisted chemical vapor deposition (PAVCD).
It appears furthermore that boron bromide is a valuable reagent as it enables deposits to be made on substrates heated to temperatures starting from 400xc2x0 C. which are low enough to prevent the clogging of the walls of the reactor.
The carrier gas for its part is used to carry the gas mixture into a reaction chamber and activate the plasma. The presence of this carrier gas consequently enables an increase in the reaction speed of decomposition of boron halogenide and therefore an increase in the speed of deposition of boron and/or boron nitride on the fiber.
Through the method according to the invention, the speed of deposition is high, in the range of 300 to 500 xcexcm/h, and compatible with the drawing speed of an optical fiber. The coating layer obtained increases the fatigue strength of the fiber, the factor n being greater than or equal to 100, without lowering its mechanical strength. The method according to the invention is furthermore a low-cost method.
Another object of the invention relates to a device for the implementation of this method. This device comprises:
a tubular reaction chamber comprising, at each of its ends respectively, an inlet lock and an outlet lock enabling the continuous passage of the optical fiber within the chamber,
a cooling system used to maintain the wall of the reaction chamber at a constant temperature equal to or below 100xc2x0 C.,
a pumping set connected firstly to the reaction chamber to trap the residual gases and maintain a constant pressure and, secondly, to the inlet and outlet locks to maintain a pressure close to or identical to the pressure prevailing inside the reaction chamber,
a gas unit connected to the inlet lock and/or to the reaction chamber, and
means for the production of microwave plasma-assisted heating.
Other special features and advantages of the invention shall appear from the following detailed description given by way of a non-restrictive illustration with reference to the appended drawings, of which: